Indwelling ureteral catheter stents or drainage tubes have been used to bypass ureteral obstructions or uretero-vaginal fistulas and to achieve and to maintain urinary drainage between the kidney and the bladder. In the past, stents made of straight lengths of open end tubing have been used for this purpose and have provided good drainage for sustained periods of time. However, the use of such tubing has not been completely satisfactory. For example, in some instances, the tubing has migrated upward and in others it has been expelled from the kidney.
Various attempts were made to produce stents which did not have the problems which accompanied the use of such tubing. For example, stents were designed which had a hook at one end to prevent downward expulsion and which had a flange at the other end to make upward migration of the stent less likely. Another approach was to provide the body of the stent with sharply pointed barbs which were designed to prevent both downward migration and expulsion from the kidney. However, such barbs increased the diameter of the stent making it more difficult to insert.
In the Finney U.S. Pat. No. 4,212,304, issued July 15, 1979, and the Finney U.S. Pat. No. 4,307,723, issued Dec. 29, 1981, ureteral stents are disclosed which are soft silicone members which have hooks at each end and which are surprisingly effective in preventing both upward migration and downward expulsion. In normal use the proximal hook is placed in the lower calyx of the kidney or the renal pelvis of the patient and the distal hook is placed in the bladder. The stent then provides a passage for urine from the kidney to the bladder. The Finney stents are widely accepted because they work well; they are well tolerated by the patients; and they can be easily introduced both endoscopically and during open surgery.
In the Densow U.S. Pat. No. 4,610,657, a modification of a Finney-type stent is disclosed which has a hook at each end, a central lumen and a reduced opening at the proximal end. The stent can be placed in a patient using a pusher wire or by the known over-the-wire technique. The guidewire system disclosed for use with the Densow stent comprises two separate guidewires. One of the wires is the pusher wire. It is smaller in diameter than the lumen of the stent, but has a proximal end which is larger than the reduced opening at the proximal end. It is used to push the stent in place when no obstructions are encountered. The other wire is used when an obstruction is encountered. It is smaller in diameter than both the first wire and the reduced opening in the stent. When an obstruction is encountered the stent and the pusher wire are withdrawn and the pusher wire is removed. The smaller diameter wire is inserted in the lumen of the stent and the stent and wire reinserted; the leading end of the smaller wire is then advanced out the reduced opening in the proximal end and maneuvered past the obstruction. The stent is then run over the wire past the obstruction. Once the leading end of the stent is past the obstruction, the stent is pushed into place with a stent pusher.
In the Carter U.S. Pat. No. 4,713,049 a guide system is disclosed which can be used with either the Finney or Densow type stents. The guide system is comprised of a relatively flexible outer member with a flexible forgiving tip and a relatively rigid movable core which fits within the outer member. Methods of inserting stents using that guide system are described in the Carter patent.
The Finney stent and the Densow modification are both made entirely of relatively soft material, such as silicone rubber, and are widely accepted because they are well tolerated, they do not migrate upwardy and they do not cause patient discomfort. However, on occasion such stents may be expelled downwardly out of the kidney. The Finney patent suggests that the hooks could be reinforced by the incorporation of "plastic, fabric, metal or other suitable material" to make them less flexible and more resistant to migration but the incorporation of such foreign materials can detract from the otherwise good memory of the stent material.
Stents have been made of stiffer less flexible material, such as polyethylene, in efforts to reduce explusion or migration, but those stents have not been as well accepted because a stent made of stiffer material can cause bladder irritation and other patient discomfort.
It would be desirable to have a stent that had all the advantages of the Finney and the Densow stents and which, in addition, would have a greater resistance to being expelled from the kidney.